In some industrial situations, as can arise, for example, in a power station or a petro-chemical installation, equipment performing vital operational functions should be provided with an uninterrupted d.c. power supply.
Hitherto, a mains-powered battery charger has been used to supply d.c. power to an equipment while maintaining a battery, connected across the output terminals of the charger, at an appropriate d.c. voltage. If the battery charger should fail power may still be supplied by the battery and so a vital operational function can be maintained.
A problem arises if the charging circuit becomes open circuit as would happen, for example, if the battery became disconnected. Since, in these circumstances, a "back-up" power supply is no longer available satisfactory operation of the equipment cannot be assured and a potentially hazardous situation may develop.